There is generally used for two-wheel drive vehicles brake benches of a type known per se, comprising two pairs of rollers adapted to be driven in the same direction by electric motors. The two wheels on the same shaft of the vehicle are advanced to position each between the two rollers of the corresponding pair; then the wheels are driven in rotation in a forward direction at low speed of the order of 5 km/hour by means of rollers driven by electric motors. The indicator means for the peripheral speed of the wheels are provided in the form of cylinders in tangential contact, driven in rotation by the wheels and provided with tachymetric detectors supplying a signal representative of the peripheral speed of the corresponding wheel. The casings of the electric motors are generally fixed to the brake bench by means of a force detector supplying a signal representative of the forces exerted on the securements of the motor casings when there is exerted a braking force on the wheels of the shaft to be controlled.
As a safety measure, the cylinders that indicate the peripheral speed of the controlled wheels are mounted on a system comprising a lever, which is actuated when a wheel is located between the corresponding drive rollers. Descending movement of this lever causes the closure of an electric contact authorizing starting of the electric drive motor of the rollers.
In known manner, the control of the braking of a two-wheel drive vehicle takes place in the following manner:
a) In a first phase in which the two wheels are simultaneously driven in the same direction at the same speed, no braking force is exerted on the brake pedal of the vehicle by the operator. The mentioned force detectors supply signals representative of the residual braking values on one or the other wheel. These residual values are generally very small and are notable in the case of poor release of the brakes or of a resistance to rotation resulting for example from a poor condition of the bearings. PA1 b) In a second phase in which the wheels on the same shaft are driven in the forward direction, a constant pressure is applied to the brake pedal by the operator. The mentioned force detectors supply values representative of the variations of braking on one and the other wheel. The variations of braking are representative of the ovalization of the wheels or of defects in the brake members: for example, a deformation of the brake drum in the case of drum brakes, or a distortion of the disc in the case of disc brakes. PA1 c) In a third phase in which the two wheels are driven simultaneously in a forward direction, a pressure is applied progressively up to a maximum value on the brake pedal by the operator. The test is halted when the relative slippage of the wheel on the drive rollers is equal to a predetermined value. The maximum braking force achieved in the course of this test is noted for each wheel of the controlled shaft. PA1 --the difference of the two speeds is maintained less than 5% of the speed of the drive means in the forward direction, PA1 --the drive means are asynchronous electric motors and the frequency of the drive motor in forward movement is adjusted as a function of the difference of the peripheral speeds of the two wheels. PA1 a) maintaining the forward drive means at a constant speed and the reverse drive means at a speed that continuously varies between a minimum speed less than said constant speed and a maximum speed greater than said constant speed, to measure in the absence of any braking the drive forces on the forwardly-driven wheel to be controlled and on the other reversely-driven wheel, PA1 b) drawing the correction curve corresponding to the difference of said drive forces as a function of the difference of the speeds of the drive means, PA1 c) measuring the respective peripheral speeds of the wheel to be controlled and the other wheel in the absence of any braking and maintaining constant the difference of the speeds of the drive means while correcting the measured forces by use of the correction curve of step b), PA1 d) calculating and recording the difference of said corrected peripheral speeds in the absence of any braking, PA1 e) continuously measuring during the braking the difference of the peripheral speeds to control the speed of the forward drive means by following a regulatory law adapted to maintain the difference of the peripheral speeds of the wheels less than a predetermined value, PA1 f) measuring the braking variations indicating an ovalization of the wheels, or measuring the braking force corresponding to a maximum value for slippage of the wheels on the rollers, PA1 g) continuously correcting the values obtained in step f) by use of the correction curve obtained in step b), which supplies the instantaneous correction values as a function of the different .DELTA. of the instantaneous peripheral speeds of the wheels. PA1 --the drive motors are electric motors supplied either by constant frequency electric current (first means), or by variable frequency electrical supply (second means), PA1 --the predetermined control law comprises a first phase of a continuous variation of the speed of the second drive motor, PA1 --the predetermined control law comprises a second phase of variation of the speed of the second drive motor, in which this speed variation is effected as a function of the difference of the peripheral speed of a first wheel and the peripheral speed of a second wheel.
The maximum braking forces are compared with parameters supplied by the manufacturer or parameters calculated from previous tests, so as to determine whether the braking is satisfactory or insufficient.
For two-wheel drive vehicles, the brake benches in which the two wheels of a controlled shaft are simultaneously driven in a forward direction provide a control process for braking and corresponding results that are entirely satisfactory.
It is not possible to use the known process and braking mode to control the braking of a four-wheel drive vehicle, in which an intermediate device for transmission of movement such as a visco coupler or an epicycloidal differential transmits a portion of the energy of the driven shaft to another shaft.
It is also known that it is impossible to use a single brake bench to cause to turn the two wheels of a same shaft in the same direction, the other shaft being in contact with the ground: thus, in the case in which the two wheels on a same shaft are driven in the same direction, it is known that the vehicle is thrown from the brake bench, even at low speed. To overcome these drawbacks, there has been envisaged:
1. mounting the first shaft of a vehicle on a control bench and mounting the second shaft on freely rotatable wheels. Such an arrangement requires, in order to be able to control the two shafts, mounting the free rollers on opposite sides of the braking control bench. However, because of unequal distribution of forces between the four rollers, there is a compensation of the forces and an artificial reduction of the dissymmetries of braking the right and left wheels of the controlled shaft.
Thus, this system does not permit giving exact values of the braking characteristics of each wheel.
2. mounting said shafts simultaneously on two control benches. However, this solution requires adjusting the spacing of the two benches as a function of the wheel base of the vehicle and regulating the speed of the four wheels constantly to be at the same value by means of a costly regulation device having limited ranges of stability.
A third solution is proposed by EP 0 236 715, which describes a process and a device for controlling automotive vehicle brakes provided with several driven shafts permanently interconnected, said vehicles having a differential for each shaft.
The wheels of at least one shaft are driven in the opposite direction by first rollers of a brake test bench, and are braked individually upon control, and the braking action of this brake on the drive of the set of rollers of this wheel is read off. In the corresponding processes and devices, the speed of rotation of the driven wheels is measured directly in the course of the braking procedure and is maintained at a same value by control of the drive of the set of rollers driving these wheels.
The fact that the speed of rotation of the wheels is directly measured during control of the brakes and is maintained at an equal value for the two wheels, has the result that there will be no rotative movement of the transmission shaft having a cardan joint kinematically connected to the shaft of these two wheels by means of the differential, and that there accordingly will not be any transmission of a couple to the other shafts rigidly coupled to this transmission shaft.
The measured speeds of rotation of the wheels of a shaft are continuously compared, either at predetermined values, or with each other, and, by regulation of the drive power of the two sets of rollers in question, are maintained at the same value. In practice, the corresponding regulation to equality of the speeds of rotation of the two driven wheels in a direction opposite to each other is delicate to effectuate: it is therefore indispensable to fix reflectors serving as indicia on the wheels, these reflectors each returning a luminous beam from a source toward a detector to supply a signal representative of the angle of rotation and of the angular velocity of rotation of each wheel.
This device can give satisfaction, but its production is costly and its use delicate. This device is only intermittently operable because soiling of the wheels during rotation dirties the optical reflectors.
Moreover, in four-wheel drive vehicles, the visco coupler or the epicycloidal differential has a starting couple which is not zero. This couple can give rise to braking counterforce, from one side to the other, equal to this couple. The document EP 0 236 715 does not provide measurement means taking account of these possible braking counterforces.